Open Jet Research Articles

Experimental study on dynamic response performance of hydrogen sensor in confined space under ceiling

With the advancement of Fuel Cell Vehicles (FCVs), detecting hydrogen leaks is critically important in facilities such as hydrogen refilling stations. Despite its significance, the dynamic response performance of hydrogen sensors in confined spaces, particularly under ceilings, has not been comprehensively assessed. This study utilizes a catalytic combustion hydrogen sensor to monitor hydrogen leaks in a confined area. It examines the effects of leak size and placement height on the distribution of hydrogen concentrations beneath the ceiling. Results indicate that hydrogen concentration rapidly decreases within a 0.5–1.0 m range below the ceiling and declines more gradually from 1.0 to 2.0 m. The study further explores the attenuation pattern of hydrogen concentration radially from the hydrogen jet under the ceiling. By normalizing the radius and concentration, it was determined that the distribution conforms to a Gaussian model, akin to that observed in open space jet flows. Utilizing this Gaussian assumption, the model is refined by incorporating an impact reflux term, thereby enhancing the accuracy of the predictive formula.

Swirl-induced hysteresis in a sudden expansion flow

Swirling sudden expansion flows are complex flow fields containing several coherent structures that depend on the swirl number and can exhibit hysteresis behavior between increasing and subsequently decreasing swirl levels. While these flows have extensively been studied in simple geometries, results involving special designed nozzles are scarce. Therefore, this paper aims to provide insights into a more complex geometry, specifically a two-step conical expansion with a converging outlet. Experimental data is acquired for changing swirl numbers at a Reynolds number in the range of 35, 000. Stereoscopic Particle Image Velocimetry is employed to characterize downstream flow structures, while Laser Doppler Velocimetry is used to characterize upstream structures and to determine the inlet swirl number. Several distinct flow patterns are found as a function of the swirl number and the identified flow patterns include, in order of increasing swirl, a Closed Jet Flow, an Open Jet Flow (OJF), and a Coandã Jet Flow (CoJF). A central positive axial velocity is noted for both OJF and CoJF downstream of the expansion due to the converging outlet geometry. At higher swirl numbers, Vortex Breakdown moves upstream into the nozzle until a negative axial velocity is noted in the inlet tube. For these higher swirl numbers, no hysteresis is observed in the inlet tube between increasing and subsequently decreasing swirl. However, downstream of the nozzle, it is observed that the CoJF detaches at a lower swirl number than the swirl number required for attachment, indicating a hysteresis effect between in- and decreasing swirl.

A simple hot wire temperature drift correction based on temperature sensitivity applied to a turbulent shear layer

A procedure is proposed to correct temperature drift for hot wire anemometry measurements in turbulent fields where the facility is not equipped with temperature control. The procedure consists of evaluating the wire sensitivity to the temperature by building a voltage-temperature curve. The voltages of both the calibration points and the measurement points are corrected, shifting the voltage values to an arbitrary reference temperature. The correction can be applied to the instantaneous voltages by performing synced temperature measurements with constant current anemometry or constant voltage anemometry cold wire. The efficacy of the method is tested on a turbulent shear layer that develops on the side of an open jet wind tunnel not equipped with temperature control. The velocity statistics show a good match with respect to reference particle image velocimetry measurements, evidencing the self-similarity of the shear layer in contrast with the non-corrected data and the data corrected using the semi-empirical and analytical relation based on King's law modification. A correction based only on the temperature statistics shows indistinguishable results with respect to the instantaneous correction.

On the enhancement of acceptable blockage of open jet wind tunnel by employing modifications based on concepts of boundary layer and jet flow for testing of aero-rotors

The conventional closed and open jet tunnels suffer from various adverse blockage effects that alter the flow conditions generating erroneous results when larger than allowable blockage models are tested. The maximum acceptable blockage is limited to 25 % for an open jet tunnel. The physics behind the inferior blockage capacity and their redressal is investigated. Hitherto, numerical models are developed to rectify the blockage troubles that introduce an additional complex case-based mathematical modeling to reckoning the results. In the current paper novel approach to redesigning the tunnel to solve blockage troubles is investigated that facilitates evading of mathematical techniques and securing accurate results straightforwardly. A hypothetical design incorporating the remedies is proposed, capable of conditioning the flow in a manner that debilitates the adverse effects of solid blockage, wake blockage, jet expansion, etc. The hypothesis is theoretically expounded by the boundary layer and jet flow phenomenon. The modified blower-type open jet wind tunnel constitutes a centrifugal blower, long inlet duct, plenum chamber around the test section, and collector. The modified wind tunnel is tested through experiments executed on four different-sized Savonius rotors creating 10 %, 30 %, 50 %, and 60 % blockage. The static pressure along the contours of the blades of rotors is obtained. The experimental results are juxtaposed with CFD results carried out at free-wind boundary conditions and standard classic experimental results performed in conventional wind tunnel at 11 % blockage. A blockage enhancement of 25 % is achieved by the approach of modification in the design of a conventional open jet wind tunnel.

Open-jet facility for bio-inspired micro-air-vehicle flight experiment at low speed and high turbulence intensity

Bio-inspired micro-air-vehicles (MAVs) usually operate in the atmospheric boundary layer at a low Reynolds number and complex wind conditions including large-scale turbulence, strong shear, and gusts. We develop an open jet facility (OJF) to meet the requirements of MAV flight experiments at very low speed and high turbulence intensity. Powered by a stage-driven fan, the OJF is capable of generating wind speeds covering 0.1 – 16.8 m/s, with a velocity ratio of 100:1. The contraction section of the OJF is designed using an adjoint-driven optimization method, resulting in a contraction ratio of 3:1 and a length-to-diameter ratio of 0.75. A modularized design of the jet nozzle can produce laminar or high-turbulence wind conditions. Flow field calibration results demonstrate that the OJF is capable of producing a high-quality baseline flow with steady airspeed as low as 0.1 m/s, uniform region around 80% of the cross-sectional test area, and turbulence intensity around 0.5%. Equipped with an optimized active grid (AG), the OJF can reproduce controllable, fully-developed turbulent wind conditions with the turbulence intensity up to 24%, energy spectrum satisfying the five-thirds power law, and the uniform region close to 70% of the cross-sectional area of the test section. The turbulence intensity, integral length scale, Kolmogorov length scale, and mean energy dissipation rate of the generated flow can be adjusted by varying the area of the triangular through-hole in the wings of the AG.

Investigating crossflow interactions in multi-perforation cooling systems: Experimental and numerical insights

This study investigates the effects of employing a multi-perforation cooling system on crossflow interactions, utilizing both numerical simulations and experimental methods. A 30° inclined multi-perforation cooling system is applied on a heated flat plate as part of the experimental setup. Large Eddy Simulation (LES) is utilized for Computational Fluid Dynamics (CFD) calculations to analyze the impact of multi-perforation on heat transfer dynamics. Validation is conducted by comparing the results with experimental investigations. The study encompasses both aerodynamic and thermal considerations, employing Particle Image Velocimetry (PIV) and infrared techniques for analysis, while wall heat flux is measured using electrical discharge. Experimental data on film cooling are collected through these methods, focusing on the impact of blowing ratio (M) and the number of open jet rows (R). The LES-based CFD simulations accurately predict the trajectory of the cooling film. Notably, the influence of flow through the multi-perforation on the primary flow is observable across both configurations of open jet rows (4R and 8R near the wall). The outcomes substantiate the formation of a cooling film emanating from the jet’s fourth row (4R configuration), thereby validating the LES simulations. Visualization of wall temperatures illustrates the formation of a cooling film, discernible through the visible heat distribution from row 4R. However, discrepancies arise in predicting the adiabatic cooling efficiency towards the exit of the 4R region and the commencement of the 8R region, potentially attributed to the alterations induced by the additional multi-perforated injection. The novelty and the main objective of this study is to evaluate the impact of the variation of geometric parameters between the jets and the main flow on the dispersion of the jets themselves. A novel function, tailored to the temperature evolution of the cooling gas, is employed for data analysis. Two geometry configurations, featuring varying open hole areas, are evaluated. The film-cooled heat transfer coefficient, depicted by the Nusselt number, and the adiabatic film cooling efficiency results, serve to appraise the film cooling efficacy of the configurations.

Furnace MILD combustion versus its open counterpart in hot coflow

The open jet flame in hot co-flow (JHC) has been frequently utilized for fundamental investigations of Moderate or Intense Low-oxygen Dilution (MILD) combustion due to its controllable conditions and relatively easy measurement capabilities. However, practical MILD combustion must take place within a combustor that is enclosed. Therefore, it is necessary to examine the similarity and disparity of combustion characteristics between two flame configurations. This issue is addressed currently. Specifically, we investigate the flow mixing, ignition, and combustion, as well as emission characteristics of non-premixed and premixed JHC and cylindrical furnace (FUR) flames at various values of the environmental temperature (Te) and central jet Reynolds number (Re). For the open non-premixed flames, we employ both previous single-tube JHC (SJHC) combustor and presently modified JHC (MJHC) one that uses the same nozzle configuration as the FUR burner. It is revealed that significant differences occur in flow, combustion and emission characteristics between the SJHC and FUR cases. On the other hand, both non-premixed and premixed MJHC configurations exhibit high similarity to the corresponding FUR cases in terms of upstream flow mixing and combustion features. Moreover, different CO and NOx emissions result from open JHC and close furnace flames due to different post-combustion configuration and residence time. Accordingly, future experiments on non-premixed MJHC and premixed JHC flames are highly recommended for better understanding practical MILD combustion.

Design, assembly and testing of an upgraded aeroacoustics wind tunnel facility

The Aeroacoustics Laboratory in the Department of Aerospace Engineering at Penn State currently serves several areas of experimental research. Most notable in the laboratory is the Anechoic Chamber of which the Aeroacoustics Wind Tunnel is an integral part. In recent years the dominant areas of research have included model aircraft exhaust jets and model rotors for helicopter applications involving detailed noise experiments. In the jet noise area, the applications have included aircraft in both flyover and take-off configurations. In the flyover application the air flow surrounding the exhaust jet has a significant effect on the radiated noise and must be simulated to produce experimental results that will be most useful in preliminary design of noise suppression applications. The Aeroacoustics Wind Tunnel at Penn State serves the purpose for this simulation. During aircraft take-off, the aircraft airspeed is in a range exceeding 250 ft per second, or Mach number of over M = 0.22. In the years preceding 2022, the velocity of the free jet flow of the Penn State facility was limited to less than 200 ft/sec. This paper reports on a project to design upgrades to the existing wind tunnel to improve the test section flow velocity to values closer to the 250 ft/sec target. The approach began with a preliminary design of the return flow ducting to convert the open jet, open return wind tunnel to a closed return tunnel. The concept is to make use of the flow energy in the exhaust of the tunnel to boost the input energy to the inlet fan that drives the flow to the test section. An integral part of the preliminary design involved making use of an upgraded flow analysis computer code to predict the gain in test section velocity of the new facility. For the available horsepower of two fans in the facility, the configurations of the existing and upgraded wind tunnel were entered into this analysis code based on a quasi-one-dimensional formulation. The code included empirical data formulated to include the various shapes of the components of the tunnels. The relative flow velocities in each section used simple incompressible continuity to calculate all velocities in the component sections. The average dynamic pressure in each section is calculated from the flow (constant) density and the velocity squared. Each section had a non-dimensional pressure loss parameter calculated from empirical data (from the relevant literature). The basic code (using the Excel algorithms) was based on a predictor-corrector concept in which the calculation was initialized with the estimated velocity and pressure of the flow at the entrance to the test section. The loss in each following section is estimated to determine the loss in the total pressure and the cumulated loss compared to the gains made at the two fans in the circuit. The initial guess is adjusted based on the total pressure value at the end of the circuit. During the related experiments, measurements of the total and dynamic pressure were performed at several joints between sections and compared to the values predicted in the analysis code. Various settings of the horsepower of the drive fans were used and the results were compared to flow measurements for the tunnel configurations (predominantly for the open return and closed return tunnel setups available before and after the upgrade project.) Results of the analysis and experiments are presented and analyzed together with the remaining activities planned for the coming months.

Experimental research on blowing-rain intensity and uniformity of rain tunnel for aircraft rain removal testing

The objective of the present study is to characterize a rain tunnel that is used for performance testing of aircraft windshield rain removal systems. The rain tunnel in the present study is an open jet wind tunnel with six full cone pressure swirl nozzles arranged at its outlet. Experiments are conducted with water at room temperature as the working medium. The influence of total water supply flow rate and air velocity on the blowing-rain intensity and uniformity is studied. The total water supply flow rate varies from 0.57 m3/h to 1.03 m3/h, while the pressure at the inlet of the nozzle ranges from 1.44 bar to 6.52 bar. The air velocity of 30 m/s, 40 m/s, and 50 m/s is studied. A special test setup has been designed for measurements of the blowing-rain intensity and uniformity. Experimental results indicate that the blowing-rain intensity and uniformity both increase with the increasing total water supply flow rate and decrease with the increasing air velocity.

An Investigation on Uncontrolled and Vortex-Generator Controlled Supersonic Jets

The present study is carried out with a motivation to investigate the axisymmetric supersonic jet both experimentally and computationally. An open jet facility was utilized to carry out the experiments, and the results were compared with computational simulations employing the K-omega SST turbulence model using ANSYS software. It is important to note that, the computational validation has been done incorporating the Rayleigh Pitot formula to match the centerline pressure for the uncontrolled jet, which has not been found in any other validation studies according to the authors’ understanding. Besides, the experimental study is extended with a focus on evaluating the impact of Vortex Generators (VGs) on Mach 1.6 supersonic jets. The aim was to enhance jet mixing, a critical factor for improving engine performance. Various nozzle geometry modifications were explored in the past, but VGs emerged as the most effective method for optimizing jet mixing efficiency. The investigation revealed a substantial decrement in the supersonic jet core length when VGs were introduced at the nozzle exit, especially under favorable pressure gradients. This reduction in the supersonic core emphasized the role of VGs in enhancing mixing efficiency. The study also confirmed that VGs significantly distort wave patterns within the supersonic core, crucial for improved jet mixing. This research signifies the importance of VGs in augmenting the mixing of Mach 1.6 jets, offering the potential for improved jet performance and reduced noise emissions in the aerospace industry.

Aeroacoustic measurements of a contra-rotating UAV vehicle

The aerodynamic sound linked to the increasing presence of Unmanned Aerial Vehicles (UAVs) is becoming a societal concern. Assessing the aeroacoustics of these vehicles is of utmost importance, as it enables the evaluation and mitigation of the associated noise emissions. An experimental wind-tunnel investigation is carried out with an octo-quad multi-rotor model UAV with 4 sets of contra-rotating propellers, representative of a heavy-lifting drone, to assess the acoustic emissions of the vehicle under different flow and thrust conditions. The experiments are performed in the Open Jet Facility of the Delft University of Technology. The acoustic measurements from several microphones are combined to create the noise signature of the UAV, while the data from a balance are processed to compute the thrust. The results of this research demonstrate that it is possible to reduce the noise emissions of the drone by varying the operating input between the fore and aft motors. In particular, an attenuation in the overall sound pressure level of 5 to 8 dBA can be achieved if the RPM of the fore motors is decreased while that of the aft motors is increased.

Suppressing Methods of the Pressure Fluctuation in Open Jet Wind Tunnels

Due to the distinctive structure of the test section, the open jet wind tunnel generates low-frequency pressure fluctuations (LFFs) within the range of typical wind speeds. These fluctuations significantly compromise the quality of the flow field in the test section. The evolution of the flow structure and vortex is analysed through the improved delayed detached eddy simulations (IDDES). The LFFs and the control mechanism in the open jet wind tunnel of Jilin University are then studied. The interaction between the large-scale vortex shedding at the nozzle exit and the collector forms the edge feedback, which is the main reason for the pressure fluctuation. According to the feedback mechanism, the LFFs are suppressed using the throat gap and by improving the collector shapes. The results show that the increase of the throat gap length at the collector can significantly alleviate the pressure accumulation inside the collector. The change of the collector shapes can control the impact area and time of the incoming flow, or produce permanent vortex structure to affect the impact shape of the vortex and the flow field at the collector, which allows to control the LFFs. This study lays a solid foundation for further comprehension of the aerodynamic characteristics of the open jet wind tunnels.

Steady and unsteady characteristics of circular arc airfoils for water pumping windmills

Accurate airfoil lift and drag data at low Reynolds number, Re, and high angles of attack, α, are needed to analyze the performance of small wind turbines, particularly their starting. In the current study, the steady and unsteady aerodynamic characteristics of circular arc airfoils (CAAs) with and without spars, as used in water-pumping windmills, were examined in a wind tunnel of 1 m2 cross section at Re<106. The tunnel was configured as an open jet and a closed section. The effects of varying geometrical characteristics on the CAA performance were investigated using a combination of thickness, camber, aspect ratio, and airfoil chord-to-tunnel height ratio. Using force transducers, the aerodynamic forces acting on the airfoils were measured directly for both increasing and decreasing α. The decreasing α measurements produce a higher lift–drag ratio than the increasing measurements, mostly in the post-stall region. In addition, “second stall,” was observed at large α in both the open and closed tunnels depending on the Re and tunnel blockage, but was much more prominent in the closed tunnel due to its wall constraining the wake and preventing the flow from switching from one regime to another. It was shown that the performance of all tested airfoils was sensitive to low Re under steady and unsteady conditions. The latter measurements for an airfoil oscillated at reduced frequencies, k≤0.06, are the first for CAAs. Results from this investigation provide a comprehensive airfoil dataset for the accurate blade element theory modeling of CAAs aerodynamic and starting performances.

Unsteady Aerodynamic Characteristics of The Flapping Wing Micro Air Vehicle (MAV)

Flapping wing propulsion is center of attention in the field of micro air vehicle research in recent years. Knowledge in unsteady aerodynamics is insufficient to design an efficient wing that produces the required lift and thrust force for the flapping wing micro air vehicle. The unsteady aerodynamic effects on aerodynamic wing design parameters were discussed in this paper. The effect of aspect ratio and the wing planform shape on lift and thrust force were determined experimentally. The experiments were conducted in an open jet low speed wind tunnel having test section 350 mmx350 mm. A rack and gear type new flapping mechanism was designed and fabricated and this was used as test bed for all the experiments. Also an experiment was conducted to determine the effect of torsional flexibility of the wing. The result shows that the chord wise flexible wing produces more lift and thrust than the other wing.

Development and Validation of a Nonlinear Fabric Model for Subsonic Parachute Aerodynamics

Parachute/flow interaction is dominant in evaluating a decelerator’s performance. Such interaction is characterized by nonlinear deformations and complex flow phenomena. While testing methods are available to investigate parachute performance, these are often costly and nonrepresentative of the desired flight conditions. To address the need for an accessible technique capable of modeling parachutes at the early design stages, this paper proposes a robust fluid/structure interaction methodology for three-dimensional subsonic simulations. This is attained by replacing the linear springs in Provot’s equation with polynomial expressions whose coefficients are fitted to tensile test data. The nonlinear cloth algorithm is coupled with the rhoPorousSimpleFoam solver in the open-source OpenFOAM toolbox, thereby establishing an iterative process that reaches steady-state convergence in at most six iterations. The transient response is obtained from the average distributed load of the steady-state pressure field and an inertial damping contribution. The simulations are performed for two disk-gap-band parachutes and a ringsail parachute over a velocity range of ring sail 5–30 m/s. The results are compared to the experimental data measured in the Open Jet Facility of Delft University of Technology, yielding errors below 5% for the steady-state cases and overestimations in peak loads of 4.4–12.4% for the transient simulations.

The Honda Automotive Laboratories of Ohio Wind Tunnel

<div class="section abstract"><div class="htmlview paragraph">The Honda Automotive Laboratories of Ohio (HALO) includes a new aeroacoustic wind tunnel located near Marysville, Ohio that started operations in 2022. This facility provides world-class aerodynamic flow quality and acoustic testing capabilities for the development of both passenger and motorsports vehicles.</div><div class="htmlview paragraph">This closed-return ¾ open jet wind tunnel features a two-position flexible nozzle system with cross sections of 25 m<sup>2</sup> and 18 m<sup>2</sup>, providing wind speeds of up to 250 km/h and 310 km/h, respectively. There is a ±180 degree turntable with boundary layer control systems, and interchangeable single belt and 5-belt moving ground plane (MGP) modules. Extensive applications of acoustic treatment in the test section and throughout the wind tunnel circuit provide a hemi-anechoic test environment and low background noise levels. A temperature control system provides uniform and stable air temperature over an operating environment between 10 °C and 50 °C.</div><div class="htmlview paragraph">The primary instrumentation of the wind tunnel includes the wind speed, temperature, and humidity measurement systems, a 6-component force balance integrated within the turntable, and an acoustic test system (ATS). The ATS includes four planar microphone arrays for external measurements and a spherical microphone array and binaural heads for internal measurements. A flow survey traverse system equipped with a 4-D probe holder mechanism is capable of placing a probe anywhere within the test volume.</div><div class="htmlview paragraph">The HALO facility includes the wind tunnel control room, secure vehicle preparation bays and office spaces for segregated customers, and a vehicle frontal and side area measurement system.</div><div class="htmlview paragraph">This paper provides an introduction to the HALO wind tunnel testing capabilities, design features and development, and the results of the aeroacoustic commissioning program. The background noise level and flow quality characteristics are provided.</div></div>

The Ford Rolling Road Wind Tunnel Facility

<div class="section abstract"><div class="htmlview paragraph">The Ford Motor Company Rolling Road Wind Tunnel (RRWT) is a state-of-the-art aerodynamic wind tunnel test facility in Allen Park, Michigan. The RRWT has operated since January 2022 and is designed for passenger and motorsport vehicle development. The test facility includes an office area, three secure customer vehicle preparation bays, a garage area, a vehicle frontal area measurement system, and a full-scale ¾ open jet wind tunnel. The wind tunnel features an interchangeable single belt and 5-belt Moving Ground Plane (MGP) system with an integrated 6-component balance, a two-position nozzle, boundary layer removal systems, and two independent flow traverse systems. Each flow traverse has a large horizontal box beam and vertical Z-strut that can position the flow traverse accurately within the test volume. The individual traverse systems can also operate in tandem and have the capability to perform stereo PIV measurements and scale model vehicle testing, including vehicle passing or drafting simulations. The MGP can be quickly changed between single belt and 5-belt operations, requiring only a single eight-hour shift to transition between the two configurations. The efficient wind tunnel airline is capable of airspeeds up to 250 km/h in the large 26.4 m<sup>2</sup> nozzle configuration and 322 km/h in the small 18.6 m<sup>2</sup> nozzle configuration using a 12-blade, 8 m diameter fan with a 5.4 MW motor. The wind tunnel’s flow quality and aerodynamic performance compare well with similar facilities in the automotive industry. Ford Motor Company is currently correlating aerodynamic measurement data from the RRWT with other similar facilities, using 60 vehicles across multiple product lines.</div></div>

Functional outcome of Latarjet for recurrent anterior shoulder dislocation

Background and Aim: Recurrent shoulder dislocations can be treated with a variety of surgical procedures. However, soft-tissue repair appears to be ineffective in all cases of recurrent shoulder dislocation. The present study aimed to determine the Latar jet technique functional outcome for recurrent anterior shoulder dislocation. Patients and Methods: This descriptive cross-sectional study was conducted on 42 recurrent shoulder dislocation patients at the MTI Lady Reading Hospital Peshawar from January 2019 to December 2022. All the eligible patients were operated with open Latar jet procedure. Constant-Murley shoulder score was used for the assessment of functional outcome at 6th months. SPSS version 27 was used for data analysis. Results: Of the total 42 patients, there were 40 (95.2%) male and 2 (4.8%) female. The overall mean age was 46.8±8.4 years. The right and left-side involvement was found in 32 (76.2%) and 12 (23.8%) respectively. The incidence of poor, fair, good, and very good outcomes was 3 (7.1%), 4 (9.5%), 12 (28.6%), and 23 (54.8%) respectively. There was significant improvement in shoulder motion and reduction in pain after 6th month's follow-up.

Flow Patterns of an Open Microchannel and Jet Impingement Hybrid Cooling Scheme

In last several decades, there are great development in electronic technology, which needs high performance cooling schemes urgently. Recently, an enhanced flow boiling of open micro-channel and jet impingement hybrid cooling scheme by separating liquid-vapor pathways is proposed with a critical heat flux of 546 W/cm2. Nevertheless, due to the introduction of jet impingement, the bubbles generated in channel are compelled to migrate to the fins top to gather and grow. Consequently, a new series of bubbles behavior and flow patterns were observed. In order to have a further understand of flow patterns and the transition mechanism, visualization experiments are conducted with the deionized water, inlet temperature is 16 - 19 °C, and volumetric flow rate for micro-channel and jet impingement is 50 ml/min respectively. Subsequently, a new phenomenon was observed, where the bubbles owned the various critical motion diameters in different flow regions. Finally, based on the force balance of a bubble on fin tops, a prediction model of critical motion diameter is also proposed. The maximum error is 20%. These findings have an impact on the further development of efficient cooling technology.

Tire dependence for the aerodynamics of yawed bicycle wheels

The aerodynamic characteristics of a modern road cycling wheel in cross wind are studied through force- and planar PIV measurements in the TU Delft Open Jet Facility. The performance of the 62 mm deep rim is evaluated for three tire profiles, and yaw angles up to 24°. All measurements are executed at 12.5 m/s (45 km/h) freestream- and wheel-rotational velocity. The wheel's rim-tire section in crosswind is found to behave similar to an airfoil at incidence, ultimately resulting in a reduction of the wheel's aerodynamic resistance with increasing yaw angle magnitude. This phenomenon, also referred to as the sail-effect, is limited by the stall angle of the tire-rim profile. The stall angle is found to depend critically on the tire's surface structure. Larger stall angles, resulting in lower resistance, are obtained if the tire profile triggers laminar-to-turbulent boundary layer transition.