Session D14: Experimental Techniques II: Aerodynamics/Wind Tunnel

Flow measurements on a low speed wind tunnel

The design and performance of a wind tunnel are tested with its characterization, that implies a detailed study of the pressure losses and the velocity profiles, which will provide knowledge of the behaviour of the flow in the tunnel. Among the results achieved, it was obtained a maximum velocity of 72 Km/h in the test section, also it was determined that its essential to implement various improvements in the design of the tunnel. It is found through the present that it is not possible to calculate the net loss in the tunnel. However, having into an account experimental parameters of the construction of the tunnel one can estimate a percentage of loss flow comparing it to the one calculated in the test section. It must also be mentioned that the losses found relative to the test section and the diffuser are negligible compared to the one relative to the contractor. This suggest a number of improvements in the design of the tunnel, mainly in the joints between sections.   

Fast response temperature and humidity sensors for measurements in high Reynolds number flows

Conventional hot/cold wires have been widely used in measuring velocity and temperature in turbulent flows due to their fine resolutions and fast response. However, for very high Reynolds number flows, limitations on the resolution appear. A very high Reynolds number flow is the atmospheric boundary layer. In order to accurately predict the energy balance at the Earth\rq s surface, one needs information about the different turbulent scalar fields, mainly temperature and humidity, which together with velocity, contribute to the turbulent fluxes away from the surface. The nano-scaled thermal anemometry probe (NSTAP) was previously developed at Princeton and has proven to have much higher spatial and temporal resolution than the regular hot wires. Here we introduce new fast-response temperature and humidity sensors that have been developed and tested. These sensors are made in-house using standard MEMS manufacturing techniques, leaving high flexibility in the process for optimization to different conditions. The small dimensions of these novel sensors enable very high spatial resolution while the small thermal mass allows significant improvements in the frequency response. These sensors have shown promising results in acquiring un-biased data of turbulent scalar and vector fields.   

Dynamic calibration and modeling of a cold wire for temperature measurement

The dynamical behavior of cold wires and their supporting structure is investigated. It is shown that conventional cold wires are slower than previously believed, which can cause substantial inaccuracies in temperature data. A new method for accurate temperature measurements using cold wires is presented. A lumped parameter model that accounts for the effects of end conduction on wire response is developed. To verify and validate the model, an experimental investigation is conducted~where the frequency response of the wire is investigated under different heating conditions. The response obtained from the model is compared to experimental results with convincing agreement. The new model can be used to correct data acquired with conventional cold wire sensors, with non-negligible end-conduction effects, so that accurate measurements can be obtained. Lastly, the new model can be used to design a new temperature sensor that has a better frequency response when compared to conventional cold-wires.   

Hot-wire based phase resolved measurement techniques for turbomachinery flows

Resolving the details of turbomachinery rotor flows from the stationary reference frame is difficult due to the high sensor frequency response required. Hot-wires have the necessary frequency response but are sensitive to both total temperature and velocity. In high-speed turbomachinery flows, the large blade-to-blade total temperature gradients prevent traditional hot-wire methods from being used to measure velocity directly. In order to decouple the effects from the temperature variation, a single constant temperature hot-wire was operated at different overheats at the exit of a high-speed transonic axial compressor rotor. The multiple overheat method was used to decouple the phase locked averages of total temperature and velocity magnitude from the phase locked average hot-wire voltages for different overheats. The phase locked average total temperature and velocity magnitude fields show flow features relative to the rotor including blade wakes, boundary layers, and tip clearance flows.   

Development of a Digital Image Projection (DIP) Technique to Quantify Wind Driven Water Droplet/Rivulet Flows over a NACA 0012 Airfoil

A digital image projection (DIP) technique is developed to achieve non-intrusive thickness measurements of wind-driven water droplet/rivulet flows. The DIP technique is based on the principle of structured light triangulation in a similar manner as a stereo vision system but replacing one of the cameras for stereo imaging with a digital projector. A grid pattern of known characteristics is projected onto a test object (i.e., the droplet/rivulet over the test plate). Due to 3D shape profile of the test object, the projected grid pattern is deformed seen from a perspective different from the projection axis. By comparing the distorted image over the test object and a reference image, the 3D profile of the test object with respect to the reference plane (i.e., the thickness distribution of the droplet/rivulet flow) can be retrieved quantitatively and instantaneously. The DIP system is used to quantify the dynamic shape change and stumbling runback motion of the wind-driven water droplet/rivulet flows over a NACA0012 airfoil. Such information is highly desirable to elucidate the underlying physics to improve our understanding about the surface water transport process pertinent to ice formation and accretion over aircraft wings in atmospheric icing conditions.   

Plasma Anemometer Measurements and Optimization

Velocity measurements using a constant-current plasma anemometer were performed in a Mach 0.4 jet in order to further optimize the anemometer design. The plasma anemometer uses an AC glow discharge (plasma) formed in the air gap between two protruding low profile electrodes as the flow sensing element. The output from the anemometer is an amplitude modulated version of the AC voltage input that contains information about the mean fluctuating velocity components. Experiments were performed to investigate the effect of the electrode gap, AC current, and AC frequency on the mean and fluctuating velocity sensitivity and repeatability of the sensor. This involved mean velocity calibrations from 0 to 140\,m/s and mean and fluctuating velocity profiles through the shear layer of the jet. Measurements with a constant temperature hot-wire anemometer were used for reference. The results showed an improvement in performance with increasing AC frequency that was attributed a more stable glow discharge. The agreement with the hot-wire were good, with the advantage of the plasma anemometer being its 100-times higher frequency response.   

Simultaneous time-resolved measurement of flow field and surface deformation combining tomographic PIV and Mach-Zehnder interferometry

Flow induced vibrations are ubiquitous in numerous applications, for which knowledge of the relationship between surface deformation and the corresponding flow field is vital to the understanding of the processes involved. A novel technique, combining tomographic PIV (TPIV) and Mach-Zehnder interferometry (MZI), has been developed to perform simultaneous, time-resolved measurements of both the 3D flow field above a complaint transparent PDMS wall, and the spatial distribution of surface deformation. Five high speed cameras are involved, four for TPIV, and the fifth for MZI. The same high-speed laser is used for both measurements by allowing a small fraction (0.1{\%}) of the TPIV light, which is transmitted through PDMS coating, to propagate through a 99.9{\%} mirror to the MZI camera. This object beam interferes with a similarly weak reference beam that does not pass through the sample volume. Methods for extracting the deformation from the resulting fringes will be discussed, such as fringe enhancement techniques to improve the S/N ratio. Sample velocity and deformation results recorded at 3kHz will be presented, demonstrating the ability of combined TPIV and MZI to study the dynamical interactions between 3D flow structure and surface deformation.   

Single Rod Vibration in Axial Flow

Fluid structure interaction of a single rod in axial flow is a coupled dynamical system present in many application including nuclear reactors, steam generators, and towed antenna arrays. Fluid-structure response can be quantified thanks to detailed experimental data where both structure and fluid responses are recorded. Such datum deepen understanding of the physics inherent to the system and provide high-dimensionality quantitative measurements to validate coupled structural and CFD codes with various level of complexity. In this work, single rods fixed on both ends in a concentric pipe, are subjected to an axial flow with Reynolds number based on hydraulic diameter of Re $=$4000 . Rods of varying material stiffness and diameter are utilized in the experiment resulting in a range of dimensionless U between 0.5 and 1, where U $=$ ($\rho $A/EI)$^{\mathrm{1/2}}$uL. Experimental measurements of the velocity field around the rod are taken with PIV from time-resolved Nd:YLF laser and a high speed CMOS camera. Three-dimensional and temporal vibration and deflection of the rod is recorded with shadowgraphy utilizing two sets of pulsed high power LED and dedicated CMOS camera. Through integration of these two diagnostics, it is possible to reconstruct the full FSI domain providing unique validation data.   

Lonely GPFUTV--the movement of water under the action of unknown vacuum

In this paper, firstly, the experiment on the flow resistance of the aerated pipe flow is introduced. The experimental research on comparison between different volumes of air entrained is presented. Secondly, the characteristics of gravity pipe flow under the action of Torricelli's vacuum, shortly called as GPFUTV are dissertated, including creative and functional design, fundamental principle, etc. Under the joint action of an unknown vacuum energy and the formation of non-aerated flow the water flow is full-pipe and continuous, high-speed and non-rotational as distinguished from turbulent flow. Thirdly, an appeal in relation to the experimental research, the applied studies and basic theory research is given. For instance, experimental~study of~Torricelli's experiment phenomenon in the vacuum environment, applied study of the potential for GPFUTV to be developed for deep seawater suction technology and lifting technology for deep ocean mining, theoretical study of flow stability and flow resistance under GPFUTV condition, etc. At last, the famous GPFUTV project is illustrated.