Blake Johnson

An Experimental Investigation on Wind-Driven Rivulet/Film Flows over a NACA0012 Airfoil by Using Digital Image Projection Technique

Aircraft icing is a serious threat to aviation safety. Icing accretion process usually interacts with surface water run back flow under glaze icing condition. In the present study, an experimental investigation was conducted to characterize the surface wind-driven water film/rivulet flows over a NACA 0012 airfoil in order to elucidate the underlying physics of the transient surface water transport behavior pertinent to aircraft icing phenomena. The experimental study was conducted in an icing research wind tunnel available at Aerospace Engineering Depratment of Iowa State University. A novel digital image projection (DIP) measurement system was developed and applied to achieve quatitative measurements of the thickness distributions of the surface water film/rivulent flow at different test conditions. The measurement results reveal clearly that, after impinged on the leading edge of the NACA0012 airfoil, the micro-sized water droplets would coalece to form a thin water film in the region near the leading edge of the airfoil. Water rivulets were found to be generated as the water film flow runs backs. The width and the spacing of the water rivulets were found to decrease monotonically with the increasing wind speed.

Measurement Uncertainty Analysis in Determining Adiabatic Film Cooling Effectiveness by Using Pressure Sensitive Paint Technique

While pressure sensitive paint (PSP) technique has been widely used to measure adiabatic film cooling effectiveness distributions on the surfaces of interest based on a mass transfer analog to traditional thermal-based measurements, very little can be found in literature to provide a comprehensive analysis on the uncertainty levels of the measured film cooling effectiveness distributions derived from PSP measurements. In the present study, a detailed analysis is performed to evaluate the effects of various associated uncertainties in the PSP measurements on the measured film cooling effectiveness distributions over the surfaces of interest. The experimental study is conducted in a low-speed wind tunnel under an isothermal condition. While airflow is used to represent the “hot” mainstream flow, an oxygen-free gas, i.e., carbon dioxide (CO2) gas with a density ratio of DR1⁄4 1.5 for the present study, is supplied to simulate the “coolant” stream for the PSP measurements to map the adiabatic film cooling effectiveness distribution over a flat test plate with an array of five cylindrical coolant holes at a span-wise spacing of three diameters center-to-center. A comprehensive analysis was carried out with focus on the measurement uncertainty and process uncertainty for the PSP measurements to determine the film cooling effectiveness distributions over the surface of interest. The final analysis indicates that the total uncertainty in the adiabatic film cooling effectiveness measurements by using the PSP technique depends strongly on the local behavior of the mixing process between the mainstream and coolant flows. The measurement uncertainty is estimated as high as 5% at the near field behind the coolant holes. In the far field away from the coolant holes, the total measurement uncertainty is found to be more uniform throughout the measurement domain and generally lower than those in the near field at about 3%. [DOI: 10.1115/1.4033506]

Effects of Flow Compressibility and Density Ratio on Film Cooling Performance

An experimental investigation was performed to examine the effects of flow compressibility and density ratio of coolant to mainstream flows on the performance of film cooling injected from a row of cylindrical holes over a flat plate. The experimental study was conducted in a transonic wind tunnel. A pressure-sensitive paint technique was used to map the corresponding adiabatic film cooling effectiveness distributions over the surface of interest, based on a mass flux analog to traditional temperature-based cooling effectiveness measurements. It was found that, at the relatively low blowing ratio of M≤0.40, the flow compressibility had almost no effects on the film cooling effectiveness over the surface of interest. At relatively high blowing ratios of M≥0.85, the film cooling effectiveness for the test case with compressible, transonic mainstream flow was found to become marginally better than that with the mainstream being incompressible. Although the density ratio was found to have limited effects on t...

An Experimental Study of Compressibility Effects on the Film Cooling Effectiveness Using PSP and PIV Techniques

() An experimental study was performed to examine effects of compressibility on film cooling effectiveness for coolant flowing from cylindrical holes in a flat plate. The incoming flow Mach number was set at 0.07, 0.30, 0.50 and 0.70. The Pressure Sensitive Paint (PSP) technique, which is based on a mass transfer analogy, was used to map the adiabatic film cooling effectiveness distribution on the surface of interest. The highresolution PSP measurement result showed that there was marginal enhancement of film cooling performance for higher Mach number flow comparing to the Ma=0.07 case. A high-resolution Particle Image Velocimetry (PIV) system was also used to conduct detailed flow field measurements to uncover the underlying physics of film cooling in the high speed flow, which showed similar flow field measurement results at M=0.4 for both Ma=0.07 and 0.30 cases, but slightly different ensemble-averaged velocity distribution was found for M=1.25 case.

Measurement Uncertainties Analysis in the Determination of Adiabatic Film Cooling Effectiveness by Using Pressure Sensitive Paint (PSP) Technique

Pressure sensitive paint (PSP) is useful for measurements of wall pressure in high speed flows, but can be used in an alternative manner in low-speed flows as a gas species concentration detector. Film cooling technology studies have been greatly aided by this use of PSP through use of a mass transfer analogy to determine the adiabatic film cooling effectiveness. The PSP technique allows measurements that have high spatial resolution at high enough sampling rate that a good statistical mean can be determined rapidly. Due to the potential of this technique to deliver high quality adiabatic effectiveness measurements, a detailed analysis of its associated uncertainty is presented herein. In this study, an ambient temperature low speed wind tunnel drives air as the main flow while carbon dioxide (CO2, DR=1.5) is used as the “coolant” gas, though the experiments are done under isothermal conditions. A detailed analysis of the technique is performed here with focus on the measurement uncertainty and process uncertainty for a film cooling study using an array of five cylindrical holes spaced across the span of a flat test plate at a spacing of three diameters center-to-center. The final analysis indicates that the total uncertainty depends strongly on the local behavior of the coolant, with near-field uncertainty as high as 5% at isolated points. In the far-field, the total uncertainty is more uniform throughout the measurement domain and generally lower, at about 3%.© 2014 ASME

An Experimental Study of Momentum-Preserving Shaped Holes for Film Cooling Using PSP and PIV

An experimental investigation was conducted to study the performance of film cooling injection from a cylindrical hole and a “W-shaped” hole. A Pressure Sensitive Paint (PSP) technique was used to map the spatial distribution of film cooling effectiveness on the surface of the test plate based on a mass-transfer analogy. The effects of mass flux ratio and density ratio on film cooling effectiveness are investigated by performing PSP experiments at various mass flux ratio using two coolant gases. In order to investigate the kinematic relationship between film cooling effectiveness and mass flux ratio, a high resolution PIV system was used to conduct flow field measurements. Interactions between the mainstream flow and injection flow are also investigated to elucidate the kinematic mechanisms of film cooling.

An Experimental Study of Heated Circular and Rectangular Jets Emitting into a Crossflow

Thermocouple measurements are performed in the spanwise–wall-normal plane at four measurement locations downstream of raised circular and rectangular stacks with the latter oriented with its major axis aligned both parallel and perpendicular to the crossflow. Exit velocity at the jet centerline is 50 m/s, exit temperature is either 425 K or 600 K, and the crossflow velocity is 10 m/s or 30 m/s. This parameter space yields blowing ratios in the range 0.89–4.14. The thermal centroid is used to characterize the plume trajectory and the spreading of the jet is determined based on a threshold temperature. The plume trajectory using this definition does not collapse cleanly to a common curve as in published studies of isothermal jets in crossflow wherein plume trajectory is defined using the velocity field, though the range of blowing ratios investigated here is relatively low compared to many other studies for which this scaling breaks down for blowing ratios less than about 4. The downstream spreading of the plumes, quantified by the core heated area versus downstream distance, shows consistent power-law behavior regardless of geometry, orientation and initial velocity and temperature conditions.