The compressible turbulent boundary layer developing over atwo-dimensional bump which leads to a supersonic pocket with aterminating shock wave has been studied. The measurements havebeen made with hot-wire anemometry and Particle ImageVelocimetry (PIV). A method to calibrate hot-wire probes in compressible ow hasbeen developed which take into account not only the ow velocitybut also the inuence of the Mach number, stagnation temperatureand uid density. The calibration unit consists of a small jetow facility, where the temperature can be varied. The hot wiresare calibrated in the potential core of the free jet. The jetemanates in a container where the static pressure can becontrolled, and thereby the gas density. The calibration methodwas verfied in the at plate zero pressure gradient turbulentboundary layer in front of the bump at three different Machnumbers, namely 0.3, 0.5 and 0.7. The profiles were alsomeasured at different static pressures in order to see theinuence of varying density. Good agreement between the profilesmeasured at different pressures, as well as with the standardlogarithmic profile was obtained. The PIV measurements of the boundary layer ow in front ofthe 2D bump showed good agreement with the velocity profilesmeasured with hotwire anemometry. The shock wave boundary layerinteraction was investigated for an inlet Mach number of 0.69.A lambda shock wave was seen on the downstream side of thebump. The velocity on both sides of the shock wave as measuredwith the PIV was in good agreement with theory. The shock wavewas found to cause boundary layer separation, which was seen asa rapid growth of the boundary layer thickness downstream theshock. However, no back ow was seen in the PIV-data, probablybecause the seeding did not give enough particles in theseparated region. The PIV data also showed that the shock wavewas oscillating, i.e. it was moving approximately 5 mm back andforth. This distance corresponds to about five boundary layerthicknesses in terms of the boundary layer upstream theshock. <b>Descriptors:</b>Fluid mechanics, compressible ow,turbulence, boundary layer, hot-wire anemometry, PIV, shockwave boundary layer interaction, shape factor. / NR 20140805

The Swedish Armed Forces setup of the Weather Research and Forecasting Model (WRF) has problems to forecast low clouds in stably stratified conditions when the ground is covered by snow. The aim of this thesis is to understand what causes this deficit. Simulations during January and February 2018 are here compared with observations from Sodankylä in northern Finland. It is revealed that neither type of planetary boundary layer parameterization chosen nor vertical or horizontal interpolation are responsible for the deficiency. Instead, our experiments show that, to first order, poor initialization of Stratocumulus (Sc) clouds from the host model, Atmospheric Model High Resolution (HRES), of the Integrated Forecast System (IFS) is the missing link. In situations when Sc clouds are missing in the IFS analysis, although they exist in reality, we use information from vertical soundings from Sodankylä. In the initialization process we used the fact that liquid potential temperature is constant in a well-mixed cloud. Initializing cloud water and cloud ice from IFS HRES and from soundings with different methods improves the model performance and the formation of very low artificial clouds at the first model level is prohibited.

The use of infrared thermography for boundary layer detection was evaluated for use in the Cal Poly Low Speed Wind Tunnel (LSWT) and recommendations for the successful use of this technique were developed. In cooperation with Joby Aviation, an infinite wing model was designed, manufactured and tested for use in the LSWT. The wing was designed around a custom airfoil profile specific for this project, where the nearly-flat pressure gradient at a zero pitch angle would delay the chordwise onset of boundary layer transition. Steady-state, RANS numerical simulations predicted the onset of transition to occur at 0.75 x/c for the design Reynolds Number condition of 6.25x105. The wing was manufactured from 3D printed aluminum, with a wall thickness of 0.125 inches and a chord length of 13.78 inches. Two central rows of static pressure taps were used, each with 12 functional chordwise locations. The taps were able to generate strong correlation to the numerically predicted pressure coefficient distribution. The use of an infrared camera visualized and confirmed the presence of boundary layer transition at the chordline location anticipated by the early simulations. To do so, the model was pre-heated such that the differential cooling properties of laminar and turbulent flow would generate a clear temperature gradient on the surface correlating to boundary layer transition. Adjustment of the model’s pitch angle demonstrated a change in the onset location of boundary layer transition during the infrared testing. The change of onset location was seen to move forward along the chordline as the aerodynamic angle of attack was increased. Testing with a Preston Tube system allowed for the interpolation of local skin friction coefficient values at each static tap location. Application of both laminar and turbulent empirical assumptions, when compared to numerical expectations, allowed for the qualitative assessment of boundary layer transition onset. Overall, the wing model developed for this research proved capable of producing quality and repetitive results for the experimental goals it was designed to meet. The model will next be used in continued tests which will further explore the use of infrared thermography.

An experimental investigation of the use low-cost microphones for unsteady total pressure measurement to detect transition from laminar to turbulent boundary layer flow has been conducted. Two small electret condenser microphones, the Knowles FG-23629 and the FG-23742, were used to measure the pressure fluctuations and considered for possible integration with an autonomous boundary layer measurement system. Procedures to determine the microphones’ maximum sound pressure levels and frequency response using an acoustic source provided by a speaker and a reference microphone. These studies showed that both microphones possess a very flat frequency response and that the max SPL of the FG-23629 is 10 Pa and the max SPL of the FG-23742 is greater than 23 Pa. Several sensor-probe configurations were developed, and the three best were evaluated in wind tunnel tests. Measurements of the total pressure spectrum, time signal, and the root-mean-square were taken in the boundary layer on a sharp-nose flat plate in the Cal Poly 2 foot by 2 foot wind tunnel at dynamic pressures ranging between 135 Pa and 1350 Pa, corresponding to freestream velocities of 15 m/s to 47 m/s. The pressure spectra were collected to assess the impact of the probe on the microphone frequency response. The two configurations with long probes showed peaks in the pressure spectra corresponding to the resonant frequencies of the probe. The root-mean-square of the pressure fluctuations did not vary much between the different probes. The root-mean-square of the pressure fluctuations collected in turbulent boundary layers were found to be 10% of the local freestream dynamic pressure and decreased to 3.5% as the freestream dynamic pressure was increased. The RMS of the pressure fluctuations taken in both laminar boundary layers and in the freestream varied between 0.5% and 2.5% of the local freestream dynamic pressure. The large difference between the RMS of the pressure fluctuations in laminar and turbulent boundary layers taken at low dynamic pressures suggests that this system is indeed capable of distinguishing between laminar and turbulent flow. The drop in the RMS of the pressure fluctuations as dynamic pressure increased is indicative of insufficient maximum sound pressure level of the microphone resulting in clipping of the pressure fluctuation; this is confirmed through inspection of the pressure time signal and spectrum. Thus, a microphone with higher maximum sound pressure level is needed for turbulence detection at higher dynamic pressures. Alternatively, it may be possible to attenuate the total pressure fluctuation signal.

No description available.

No description available.

The atmospheric boundary layer (ABL) is the lowest layer of the atmosphere where surface effects are felt on time scales of about an hour. While its properties are determined by the surface characteristics, season and synoptic conditions, they in turn determine convective cloud properties and are required for the representation of cloud processes in atmospheric models. Further, interaction of the ABL with the surface layer of the ocean is a key component of ocean-atmosphere coupling. ABL characteristics over ocean surrounding the sub-continent become very important for understanding the monsoon processes during the monsoon season because the roots of many monsoon systems, that give rain to India, are over there. In this thesis data used are from three major field experiments namely the Bay of Bengal Monsoon Experiment (BOBMEX, 1999), Arabian Sea Monsoon Experiment (ARMEX, in two phases, ARMEX-I during 2002 and ARMEX-II in 2003), and Continental Tropical Convergence Zone (CTCZ) experiment (Pilot in 2009) which were carried out under the Indian Climate Research Programme (ICRP). While there have been few studies on ABL characteristics for individual cruises, a comprehensive study considering all available radiosonde data from the above cruises has been missing. This study fills this gap and focuses on the vertical structure of ABL using more than 400 high resolution Vaisala GPS radiosonde data collected over Bay of Bengal and Arabian Sea. The study attempts at first to look at the ABL characteristics of individual cruises and then compare and contrast them over the Bay of Bengal and Arabian Sea. ABL height Hm, estimated by using virtual potential temperature (θv) profile, shows diurnal variation during weak phase of convection while maximum in early morning during active phase of convection. Different variables i.e. moist static energy (h), specific humidity (q), convective available potential energy (CAPE), virtual potential temperature (θv) and equivalent potential temperature (θe) also differ during weak and active convection periods. Conserved variables mixing line approach gives the height up to which ground thermals penetrate in the vertical. This height, denoted by MH that represents the actual ABL height, is 2-3 times larger than Hm when shallow convective clouds are present. In general both Hm and MH are 20-30% larger over Arabian Sea compares to that over Bay of Bengal. Comparison of surface convective available potential energy (CAPE) and equivalent potential temperature (θe) between normal and deficit monsoon years shows that convective instability was as large in deficit years. This means that dynamic and not thermodynamics, controlled the occurrence of convection.

Despite years of research in high speed boundary layer flow, there is still a need for insightful experiments to realize key features of the flow like boundary layer response to different conditions and related transition mechanisms. Volumes of data on the these problems point to the fact that there is still much to be understood about the nature of boundary layer instability causing transition and growth of boundary layer in different conditions. Boundary layer stability experiments have been found to be more useful, in which the boundary layer is perturbed and its behavior observed to infer useful conclusions. Also, apart from the stability part, the effect of various changes in boundary layer due to the perturbation makes interesting observation to gain more insight into the understood and the not so understood facets of the same. In view of the above, the effect of a steady axisymmetric thermal bump is investigated on a hypersonic boundary layer over a 60º sharp cone cylinder model. The thermal bump, placed near tip of the cone, perturbs the boundary layer, the behavior of which is observed by recording the wall heat flux on the cone and cylinder surface using platinum thin film sensors. The state of the boundary layer is qualitatively assessed by the wall heat flux comparisons between laminar and turbulent values. The same thermal bump also acts as a heat addition source to boundary layer in which case this recorded data provides a look into the effect of the heat addition to the wall heat flux. To gain a larger view of heat addition causing changes to the flow, effects of change in enthalpy are also considered. Experiments are performed in the IISc HST2 shock tunnel facility at 2MJkg−1 stag-nation enthalpy and Mach number of 8,with and without the thermal bump to form comparisons. Some experiments are also performed in the IISc HST3 free piston driven shock tunnel facility at 6MJkg−1, to investigate the effect of change in stagnation enthalpy on the wall heat flux. To support the experimental results theoretical comparisons and computational studies have also been carried out. The results of experiments show that the laminar boundary layer over the whole model remains laminar even when perturbed by the thermal bump. The wall heat flux measurements show change on the cone part where there seems to be fluctuation in the temperature gradients caused by the thermal bump, which decrease at first and then show an increase towards the base of the cone. The cylinder part remains the same with and without the thermal bump, indicating heavy damping effects by the expansion fan at cone cylinder junction. A local peak in wall heat flux is observed at the junction which is reduced by 64% by the action of the thermal bump. The possible reason for this is attributed to the increased temperature gradients at the wall due to delayed dissipation of heat that is accumulated in the boundary layer as a result of the thermal bump action. The comparison of data for enthalpies of 2MJkg−1 and 6MJkg−1 show that there are negligible real gas effects in the higher enthalpy case and they do not affect the wall heat flux much. Also it is found that the thermal bump fails to dump heat into the flow directly though it creates heat addition virtually by mere discontinuity in the surface temperature and causes temperature gradients fluctuation in the boundary layer. Considering the thermal bump action and the change in stagnation enthalpy of the flow, there seems to be no change in both cases that can be attributed to a common observation resulting from the factor of change in heat inside the boundary layer.

The object of the present work is to experimentally study the case of a turbulent boundary layer subjected to an Adverse Pressure Gradient (APG) with separation and reattachment. The effect of unsteadiness on turbulent boundary layer separation by means two different methods were explored viz. the effect of local forcing by acoustic waves and effect of wakes on separation bubble. The experiments were conducted in a low speed open circuit blower type wind tunnel. The turbulent separation bubble was created on the test plate by a contoured ceiling which created the adverse pressure gradient. The velocities were measured using single element hot wire and X-wire. Limited studies on quasi shear stress were also conducted using surface mounted hot film probes. Static pressure was measured using a projection manometer. Boundary layer is tripped near the leading edge of the flat plate to ensure a turbulent boundary layer. Surface pressure distribution and flow visualization were conducted as part of diagnostics. In the case of laminar separation bubble, lot of investigations have been done on the effect of unsteady wake and the most important conclusion was that the wake induces `bypass' transition to turbulence and since the turbulent boundary layer is more resistant to separation, it remains attached. In the case of turbulent separation bubble, laminar-turbulent transition is not relevant and if the bubble is suppressed, it should be by some other mechanism. This is what we seek to unravel in this study. A closer look at the mean velocity profiles reveal the occurrence of inflection point before separation as in the case of laminar separation bubble and the peak values of turbulence intensities correspond to the location of point of inflection. Turbulent separation correlations proposed by various investigators were compared with the present results and are found to be in good agreement. Surface flow visualization pictures are used to get qualitative information. The wall forcing on the separation bubble was done using a speaker which blows a small amount of air when the diaphragm moves up and sucks in when the diaphragm moves down. The blowing effect seems to be more effective in suppressing the separation compared to suction. The interaction with wake is studied using an unsteady bar which is moving up and down. The inflection point in the mean velocity distribution seems to move closer to the wall with the impingement o the wake. Also the turbulence intensities have increased and seem to move closer to the wall. The displacement and momentum thickness have increased and the shape factor has decreased which indicates suppression of the bubble. The quasi shear stress in the separated region also increased which indicates suppression of separation. While the oncoming unsteady wake might be a parcel of fluid with defect velocity when seen in isolation, in comparison to the velocity defect in the separation bubble, it is a region of velocity excess. As a result, one can expect the impingement of the unsteady wake on the TSB to transport momentum thereby contributing to separation reduction. But the mechanism of separation is different from laminar separation bubble affected by wakes. The suppression in the case of turbulent separation bubble is partly due to the entrainment of turbulence and partly due to the kinematic impact of the wake on the bubble.

Approved for public release, distribution is unlimited / This thesis study focuses on understanding the dissipation processes of the stratocumulus deck after sunrise. This objective is met through careful analyses of observational data as well as model simulations. Measurements from the Marine Atmosphere Measurement Lab (MAML) of the Naval Postgraduate School (NPS) are used in this study. In particular, the half-hourly wind profiler/Radio Acoustic Sounding System (RASS) measurements were used to determine the boundary layer top and the evolution of the boundary layer mean thermodynamic properties during the cloud breakup period. Measurements from a laser ceilometer and the routine surface measurements are also used to detect the variation of cloud base height, the evolution of the cloud deck, and the onset of sea breeze. These measurements revealed the increase of the boundary layer depth after sunrise followed by a decrease of the boundary layer depth after the onset of the sea breeze, which points to the role of surface heating and sea breeze development in modulating cloud evolution. The effects of surface heating and sea breeze are further tested using a 1-dimensional mixed layer model modified for coastal land surfaces. / Lieutenant Commander, United States Navy