Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session Q20: Experimental Techniques: Aerodynamics / Wind Tunnel |
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Chair: Joanna Austin, California Institute of Technology Room: 602 |
Tuesday, November 26, 2019 7:45AM - 7:58AM |
Q20.00001: ABSTRACT WITHDRAWN |
Tuesday, November 26, 2019 7:58AM - 8:11AM |
Q20.00002: Estimation of wall-shear stress and its variation in a unperturbed and perturbed plane wall-jet Sravan Artham, Shibani Bhatt, Zheng Zhang, Ebenezer Gnanamanickam Artham Sravan, Shibani Bhatt and Ebenezer Gnanamanickam Measurement of the wall shear stress (WSS) were conducted using near-wall hot-wire anemometry based velocity measurements and a momentum integral based measurement using time-resolved particle image velocimetry (PIV) velocity fields. Measurements were carried out on a perturbed and unperturbed plane wall jet (PWJ) developing in still air. The plane wall jet was acoustically excited at frequencies from 1 to 20 Hz which are large-scale perturbations to the flow. Measurements from a single hot-wire were carried out in the near-wall region extending into the viscous sublayer at several streamwise locations extending from $x/b=110$ to $162$. Here $x$ is the streamwise distance from the PWJ exit and $b$ the nozzle slot width. These measurements were then curve-fit with direct numerical simulation based velocity profiles to simultaneously extract the WSS as well as a correction to the wall location. The integral method used a momentum integral approach (Mehdi, F. et al.,Exp. Fluids, 2014) where PIV based velocity profiles of the streamwise and wall-normal velocities are integrated to determine the WSS. A reduction in WSS was observed for all the cases considered. The two techniques are compared and key differences are highlighted. [Preview Abstract] |
Tuesday, November 26, 2019 8:11AM - 8:24AM |
Q20.00003: Smoke visualization over a spinning cone at angle of attack in flight Abdullah Kuraan, \"Omer Sava\c{s} Smoke visualization over a spinning truncated cone at angle of attack is carried out to study qualitatively the salient features of the flow field. A twisted wire pair is used for generating equally spaced smoke streaks in vertical planes. Detailed images of flow patterns over a spinning cone with a half angle $\theta_c = 30^\circ$ and a base diameter of $20\,cm$ are captured with a high speed camera. The rotational speed of the cone is set at $\Omega = 5\,revs/s$, the axial flow at $U_\infty = 1\,m/s$, and the angles of attack up to $36^\circ$. The Reynolds number, based on the cone size, is $\sim 10^4$. The analytic description of the axisymmetric potential flow over a fixed cone at zero incidence is reviewed for comparison of the flow field outside the boundary layer. Good agreement in the streakline patterns between experiment and theory is observed near the tip of the cone at zero incidence for both fixed and rotating cases. At angle of attack, the streakline patterns outside the boundary layers are similar for both fixed and rotating cases, including evidence of vortex formation. Streakline patterns in the boundary layers reflect the effect of rotation of the surface. [Preview Abstract] |
Tuesday, November 26, 2019 8:24AM - 8:37AM |
Q20.00004: Nitric Oxide Spectroscopic Measurements of a Shock-Boundary Layer Interaction in Hypervelocity Flow Nelson Yanes, Joanna Austin Spatially resolved emission spectra are collected in the post bow shock and reattachment shock region of hypervelocity flow over a double wedge. The Hypervelocity Expansion Tube (HET) is used to generate high Mach number, high enthalpy flow (M $=7$, $h_{0}=8$ MJ/kg) over a 30-55 degree double wedge. The NO $\gamma$ band ($A^{2}\Sigma^{+} - X^{2}\Pi$) emission is measured in the UV range of 210-250 nm. Detector exposures occur at select times throughout the flow development process to study temporal changes in thermal and chemical non-equilibrium. Profiles of vibrational band intensity and spectra as a function of distance are provided. Through a fitting procedure, the experimental spectra are matched with synthetic spectra to obtain an estimate of the excitation temperature of the NO molecule. The result is a temperature profile of the post-shock NO* with downstream distance. [Preview Abstract] |
Tuesday, November 26, 2019 8:37AM - 8:50AM |
Q20.00005: Fiber-Optic Michelson Interferometer System for Shock Speed Measurement in an Expansion Tube Wesley Yu, Joanna Austin Shock speed and time of arrival measurements are necessary to properly characterize the flow in high-enthalpy hypersonic impulse facilities. However, the low density test conditions in expansion tubes present challenges for commonly-used wall-mounted static pressure measurements due to low signal levels relative to the amplitude of stress waves resulting from diaphragm rupture or accelerations from driver operation, and an accurate measurement of shock speed is difficult. A fiber-optic Michelson interferometer architecture is being designed and tested for use in shock speed measurement in the Hypervelocity Expansion Tube (HET) at Caltech. The static and dynamic response of the interferometer is characterized using a pressure vessel and ultrasonic acoustic beam, respectively. Numerical simulations of the interferometer response to a shock wave at experimental conditions, taking into account the structural noise caused by diaphragm opening, indicate that adequate signal-to-noise ratios may be obtained using this technique. This result is encouraging for the development of optical methods to characterize shock speed in impulse facilities with recoil. [Preview Abstract] |
Tuesday, November 26, 2019 8:50AM - 9:03AM |
Q20.00006: Fan array wind tunnels: turbulence on-demand Christopher Dougherty, Alejandro Stefan-Zavala, Peter Renn, Morteza Gharib Naturally-occurring winds are distinctly unsteady and non-uniform, particularly in complex urban airscapes. To better understand flight in these contexts, it is imperative to be able to control wind profiles in a predictable, repeatable, and representative manner. This can effectively be accomplished with fan array wind tunnels (FAWT), a modularly built multi-source wind tunnel capable of generating a host of spatiotemporally-varying flows via software interfacing. Utilizing an array of DC-powered off-the-shelf cooling fans (in place of one singular drive section) allows for greater flow control, overall decreased mixing lengths, and comparably large useable test section areas when compared with its effective footprint. Tunnel resolution along with fan responsiveness determines the effectiveness of one of its most salient features: the ability to tailor turbulence around a desired a mean (i.e. turbulence on-demand). This decoupling adds new territories to explore in simulated flight contexts. Periodic and unsteady flows will be highlighted with brief considerations given to the future role of machine learning. [Preview Abstract] |
Tuesday, November 26, 2019 9:03AM - 9:16AM |
Q20.00007: Flow shaping in wind tunnels with fan array technology Guillaume Catry, Nicolas Bosson, Geshanth Visvaratnam, Flavio Noca In the past hundred years, wind tunnels have been built with the goal of generating uniform flows. In particular, the geometry of the contraction and the diffuser walls has to be carefully designed in order to achieve flat profiles in the test section and avoid boundary layer separation (both in the contraction and the diffuser). The resulting infrastructure has a large footprint and is generally unmodifiable during the whole the lifetime of the wind tunnel. We have developed a technology to shape the morphology of wind in space and time. It is based on a large number of fans (wind pixels), which are distributed arbitrarily in space and can be modulated individually. In particular, we show how this technology allows the wind profile in a test section to be controllable and does not require any {\sl a priori} design of complex wind tunnel infrastructure [Preview Abstract] |
Tuesday, November 26, 2019 9:16AM - 9:29AM |
Q20.00008: A low-turbulence transverse gust generator in a wind tunnel David Olson, Ahmed Naguib, Manoochehr Koochesfahani There exists a broad range of aerodynamic problems where the commonly used steady uniform freestream condition is not appropriate. Airfoil-gust interactions are one such problem, with the transverse gust being particularly difficult to study experimentally. We present a novel transverse gust generator consisting of an actuated array of vortex generators mounted to a wind tunnel's test section. The primary advantage of the design over existing gust generators is its capability to produce a reasonably-uniform transverse stream without producing turbulence in the freestream. The generator's design can potentially allow for the time history control of the magnitude, direction, and duration of the gust strength. A simplified model for the performance of the design, and the experimental characterization of the gust generator are discussed. [Preview Abstract] |
Tuesday, November 26, 2019 9:29AM - 9:42AM |
Q20.00009: Determination of drying and shrinkage characteristics of porous food material in convective drying: Model development and experimental studies Punit Singh, Prabal Talukdar Food materials are dried to enhance the shelf life, maintain nutritional value, lower packing cost, and reduce shipping cost. An innovative convective dryer is developed, which is similar to a small scale wind tunnel, to determine the drying and shrinkage characteristics of porous food material. A conjugate 3D numerical model is also developed in a commercial software COMSOL to determine the heat and moisture transfer distribution in the food material. The model considers several important phenomena like surface evaporation, internal evaporation, and shrinkage during the drying process to render high accuracy. Results show that the air temperature has significant effect on both drying and shrinkage characteristics as compared to the airflow velocity. Around 80$\% $of moisture is removed in the first 30$\% $of time and remaining 20$\% $of moisture content is removed in the last 70$\% $of time, because of high moisture gradient present in the beginning of drying. The bulk density of food increases initially, reaches a maximum value and finally decreases in the end of drying process. Numerical studies performed with this model will help to design and develop cold storage, estimate the processing time of various operations like cooling and heating. [Preview Abstract] |
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