Douglas Bohl

MTV measurements of the vortical field in the wake of an airfoil oscillating at high reduced frequency

We present an experimental investigation of the flow structure and vorticity field in the wake of a NACA-0012 airfoil pitching sinusoidally at small amplitude and high reduced frequencies. Molecular tagging velocimetry is used to quantify the characteristics of the vortex array (circulation, peak vorticity, core size, spatial arrangement) and its downstream evolution over the first chord length as a function of reduced frequency. The measured mean and fluctuating velocity fields are used to estimate the mean force on the airfoil and explore the connection between flow structure and thrust generation. Results show that strong concentrated vortices form very rapidly within the first wavelength of oscillation and exhibit interesting dynamics that depend on oscillation frequency. With increasing reduced frequency the transverse alignment of the vortex array changes from an orientation corresponding to velocity deficit (wake profile) to one with velocity excess (reverse Karman street with jet profile). It is found, however, that the switch in the vortex array orientation does not coincide with the condition for crossover from drag to thrust. The mean force is estimated from a more complete control volume analysis, which takes into account the streamwise velocity fluctuations and the pressure term. Results clearly show that neglecting these terms can lead to a large overestimation of the mean force in strongly fluctuating velocity fields that are characteristic of airfoils executing highly unsteady motions. Our measurements show a decrease in the peak vorticity, as the vortices convect downstream, by an amount that is more than can be attributed to viscous diffusion. It is found that the presence of small levels of axial velocity gradients within the vortex cores, levels that can be difficult to measure experimentally, can lead to a measurable decrease in the peak vorticity even at the centre of the flow facility in a flow that is expected to be primarily two-dimensional.

Near Exit Plane Effects Caused by Primary and Primary-Plus-Secondary Tabs

Triangular-shaped tabs, attached to the separation lip of a nozzle, have been shown to provide substantial increases in the convective transport-mixing in a jets near field. Detailed information is reported on the basic features of such tab flows made available by 1) the large-scale tab and 2) the finely resolved pressure/velocity measurements. Secondary tabs were placed next to, and pointed away from, the primary tab; the motivation was to enhance the outward directed flow from the jets core region. The strong mixing enhancement, provided by these optimally oriented (pitch angle = 40 deg) secondary tabs, is detailed

Pulse width modulated constant temperature anemometer

A constant temperature anemometer utilizes a fine wire or thin film sensor or probe and a pulse width modulated driver in a digital control loop to adjustably drive the probe to maintain its temperature at a nominally constant value and provide a digital output to associated metering or recording equipment. The pulse width modulated driver includes a clock, an RS flip-flop and a comparator. At the beginning of the clock cycle, power is provided to a voltage divider and the divided voltage drives one input of the comparator and the fine wire or thin film sensor. The other input of the comparator is maintained at a reference voltage. When the drive voltage to the hot wire or thin film sensor exceeds the reference voltage, the comparator provides a high logic signal to the RS flip-flop, resetting it and shutting off power to the sensor until the next clock cycle. The pulse width modulated drive signal may be utilized directly by an associated processor or computer. Various circuit features may be added to improve operation given specific installation or operating parameters.

Flow visualization of a vortex ring interaction with porous surfaces

The interaction of vortex rings of constant Reynolds number with porous surfaces composed of wire meshes of constant open area, i.e., surface porosity, but variable wire diameter is studied using flow visualization. The results indicate that several regimes of flow behavior exist in the parameter space investigated. The vortex ring passes through and immediately reforms downstream of the surface for porous surfaces with small wire mesh diameters. The transmitted vortex ring has the same diameter, but lower convection speed and circulation than the pre-interaction vortex ring. For these cases, secondary vortex rings are formed on the upstream side of the porous surface that convect upstream away from the screen. As the wire diameter of the porous surface is increased, smaller sub-scale vortical structures are formed on the transmitted vortex ring as it passes through the surface. The spatial scale of these structures is dependent on the diameter of the mesh wire. The vortex ring is disrupted but is able to...

Reactive Materials Studies

Three experimental techniques have been used to investigate the impact ignition of reactive materials. The three techniques are direct impact, indirect impact, and two‐step impact. For the first two techniques, time‐resolved light spectroscopy was used to identify reaction species from solid PTFE/Al reactive material. A common observation for these two techniques is that heating and some reaction was observed during initial impact of the PTFE/Al reactive material but the majority of the reaction appeared to occur following material breakup and subsequent impact with a secondary surface. There was no spectral evidence for aluminum‐fluorine combustion. For the two‐step impact technique, the reactive material was initially pulverized as it passed through a thin plate and then subsequently ignited when the debris cloud impacted a rigid anvil. All three experiments were observed with high‐speed photography.

Experiments With Three-Dimensional Passive Flow Control Devices on Low-Pressure Turbine Airfoils

The effectiveness of three dimensional passive devices for flow control on low pressure turbine airfoils was investigated experimentally. A row of small cylinders was placed at the pressure minimum on the suction side of a typical airfoil. Cases with Reynolds numbers ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) were considered under low freestream turbulence conditions. Streamwise pressure profiles and velocity profiles near the trailing edge were documented. Without flow control a separation bubble was present, and at the lower Reynolds numbers the bubble did not close. Cylinders with two different heights and a wide range of spanwise spacings were considered. Reattachment moved upstream as the cylinder height was increased or the spacing was decreased. If the spanwise spacing was sufficiently small, the flow at the trailing edge was essentially uniform across the span. The cylinder size and spacing could be optimized to minimize losses at a given Reynolds number, but cylinders optimized for low Reynolds number conditions caused increased losses at high Reynolds numbers. The effectiveness of two-dimensional bars had been studied previously under the same flow conditions. The cylinders were not as effective for maintaining low losses over a range of Reynolds numbers as the bars.Copyright

Molecular tagging velocimetry measurements of axial flow in a concentrated vortex core

The characteristics of the axial flow within the core of a concentrated line vortex are investigated using molecular tagging velocimetry (MTV). A well-defined array of isolated vortices of alternating sign is generated in the wake of a NACA-0012 airfoil pitching sinusoidally at small amplitude and high reduced frequencies. The circulation and peak vorticity of the vortices are varied by the choice of oscillation frequency. Interaction of these two-dimensional vortices with the walls of the test section generates an axial flow within the vortex cores. The magnitude of the axial flow and its spatial/temporal characteristics are quantified using the MTV technique. Results show that the peak axial flow speeds can be very high, of the order maximum swirl speed of the vortices. The maximum axial speed ratio (maximum axial speed normalized by maximum swirl speed) is found to vary in the range 0.6–1.0 for the parameters investigated here. Initially, the axial flow is spatially confined in isolated structures corr...

Experimental Investigation of the Fluid Motion in a Cylinder Driven by a Flat-Plate Impeller

The flow field in a cylindrical container driven by aflat-bladed impeller was investigated using particle image velocimetry (PIV). A range of Reynolds numbers (0.005-7200), based on the container radius r W , were investigated using four Newtonian fluids: water (Re=7200,6800), 85/15 glycerin/water mixture (Re=108), pure glycerin (Re=8), and corn syrup (Re=0.02,0.005). Two impellers with a radius of 0.43r w , and 0.95r w were used to drive the flow. The 0.43r w , impeller was shown to generate a vortex near the tip of the blades. The peak magnitude of the vortices and the size of the vortices in the radial direction decreased with increasing Reynolds number Additionally, the vortex generated at the high Reynolds number was unsteady with a trailing shear layer that periodically shed vorticity into the flow field. The structure of the flow in the region between the blade and the cylinder wall showed a Reynolds number dependence, though the two lowest Reynolds number (0.02 and 8) flows investigated had quantitatively similar flow structures. These cases were found to have a closed region of reverse flow between the blade tip and the cylinder wall. No recirculating flow was indicated for the Re =108 and 7200 cases. These data indicate that there may be a critical condition below which there is little dependence in the flow structure on the Reynolds number.

SEPARATED FLOW TRANSITION MECHANISM AND PREDICTION WITH HIGH AND LOW FREESTREAM TURBULENCE UNDER LOW PRESSURE TURBINE CONDITIONS

A correlation for separated flow transition has been developed for boundary layers subject to initial acceleration followed by an unfavorable pressure gradient. The correlation is based on the measured growth of small disturbances in the pre-transitional boundary layer. These disturbances were identified and quantified through spectral analysis of the wall normal component of velocity. Cases typical of low pressure turbine airfoil conditions, with Reynolds numbers (Re) ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) were considered at low (0.5%) and high (8.7% inlet) freestream turbulence levels. In some cases, two-dimensional rectangular bars were placed at the beginning of the adverse pressure gradient region as passive flow control devices. The dimensionless magnitude of the initial disturbance which begins to grow at the suction peak depends on the freestream turbulence level and the size of any bar applied to the surface. The growth rate depends on the Reynolds number. When the pre-transitional disturbances grow to a sufficient magnitude, transition begins. The new correlation is based on the physics observed in the turbulence spectra, but allows transition prediction using only the Reynolds number, freestream turbulence level and bar height. The correlation has been checked against experimental data from the literature, and allows transition location prediction to within the uncertainty of the experimental measurements. The correlation represents an improvement over previous correlations which accounted for Reynolds number or freestream turbulence effects, but not both.Copyright

ANALYSIS AND MODELING OF AN EXPERIMENTAL DEVICE BY FINITE-TIME LYAPUNOV EXPONENT METHOD

In this paper, we investigate the transport and mixing process of the batch mixers with two different configurations, the centered-blade and offset-blade mixers, by using a dynamical system approach. The 2-D velocity fields of the mixers measured using Particle Image Velocimetry (PIV) are used to identify the Lagrangian coherent structures (LCSs). The results show that the LCSs separate the physical space into two portions. In the case of the center-blade mixer the portion bounded inside the LCS experiences a relatively slow mixing relative to the portion outside of the LCS boundary. However, when the blade position is located near the wall, the LCS becomes more complicated but it still separates regions of fast mixing from a slower one. We develop a heuristic dynamical system model of our mixers to understand how the vorticity strength at the blade tips influences the variation of the LCSs. Finally, we define an appropriate notion of mixing to study the mixing rate of our mixing devices.