Petar Vukoslavcevic

A 12-sensor hot-wire probe to measure the velocity and vorticity vectors in turbulent flow

A 12-sensor hot-wire probe, capable of simultaneously measuring the velocity and vorticity vectors, is described. The probe is a further step in vorticity and velocity hot-wire measurement technique development, extending the nine-sensor hot-wire probe. Results from tests of the probe performance are presented to show that the velocity field is measured with sufficient accuracy in order to determine the velocity gradients and, therefore, the vorticity vector field. The probe also has been tested in a turbulent boundary layer in order to compare its performance with that of the nine-sensor probe. The advantage of this new probe, compared with the nine-sensor probe, is clearly demonstrated for flow fields with a high ratio of cross stream to streamwise velocity components, as well as for the measurement of higher statistical moments of the turbulence.

The simultaneous measurement of velocity and temperature in heated turbulent air flow using thermal anemometry

A probe made up of one cold and two hot-wire sensors, to simultaneously measure the temperature and velocity fluctuations, has been designed and analysed. In order to optimize the probes geometrical configuration and reduce mutual interference of the sensors, several configurations were constructed and tested. The optimal position of the cold sensor and its separation from the hot one has been determined for different flow conditions. The choice of optimal operational parameters, i.e. the current through the cold sensor and the overheat ratio of the hot sensors, has also been analysed. A mathematical model, describing the simultaneous influence of speed and temperature on both cold and hot sensors, is proposed and a data reduction procedure developed. The probe was tested in nominally irrotational flow by simultaneously varying the magnitude of the flow speed and temperature as well as the flow direction. The behaviour of the probe in the most critical condition, i.e. low speed around 1 m s−1, was also determined.

An analytical approach to the uniqueness problem of hot-wire probes to measure simultaneously three velocity components

The uniqueness range of hot-wire probes to simultaneously measure all three velocity components is analysed analytically. Expressions defining the uniqueness cone half-angle are presented for triple as well as for quadruple sensor configurations. Using these expressions the influence of the main probe parameters on the uniqueness cone, i.e. the number of sensors and their orientation, the effective angle and the pitch and yaw coefficients, are analysed. Different types of probes are compared and optimum parameters are proposed. The same analysis is valid for complex vorticity probes configured as combinations of triple and quadruple sensor arrays.

Enlarging the uniqueness cone of the nine-sensor, T-configuration probe to measure the velocity vector and the velocity gradient tensor

The original concept of a nine-sensor T-configuration hot-wire probe to simultaneously measure three velocity and three vorticity components, as well as other velocity gradient-based turbulence properties, assumes three T-arrays, each with three sensors inclined at 45° to the probe axis. It is shown in this paper that probes having sensors asymmetrically arranged with respect to the probe axis, at specific but different geometrical angles, provide larger angular uniqueness domains in comparison to the original probe. To date it was believed that orthogonal arrays in which three sensors are mounted at 54.74° to the probe axis, as well as their vorticity counterparts, have the largest uniqueness domain of all triple-array-based probes. However, the proposed modified design of the T-array provides uniqueness cone half-angles similar to those that can be achieved by the orthogonal triple-array. In more extreme cases (i.e. at larger departures of the sensor angles from 45°), the uniqueness range of the modified T-array can be made even larger in comparison to the orthogonal array.

Using direct numerical simulation to analyze and improve hot-wire probe sensor and array configurations for simultaneous measurement of the velocity vector and the velocity gradient tensor

Multi-sensor, hot-wire probes of various configurations have been used for 25 years to simultaneously measure the velocity vector and the velocity gradient tensor in turbulent flows. This is the same period in which direct numerical simulations (DNS) were carried out to investigate these flows. Using the first DNS of a turbulent boundary layer, Moin and Spalart [“Contributions of numerical simulation data bases to the physics, modeling and measurement of turbulence,” NASA Technical Memorandum 100022 (1987)] examined, virtually, the performance of a two-sensor X-array probe with the sensors idealized as points in the numerical grid. Subsequently, several investigators have used DNS for similar studies. In this paper we use a highly resolved minimal channel flow DNS, following Jimenez and Moin [“The minimal flow unit in near-wall turbulence,” J. Fluid Mech. 225, 213 (1991)], to study the performance of an 11-sensor probe. Our previous studies of this type have indicated that, on balance, a probe of the desi...

On the accuracy of measurement of turbulent velocity gradient statistics with hot-wire probes

A very high resolution minimal channel flow DNS was used to examine, virtually, the ability of various multi-sensor hot-wire probe configurations to measure the statistics of velocity gradient components. Various array and sensor configurations and the spatial resolution of probes with these configurations were studied, building on designs and investigations of various authors. In contrast to our previous studies, which focused on turbulent vorticity, vorticity-velocity correlations, dissipation and production rate, here the measurement accuracy of each component of the velocity vector gradient tensor is analyzed separately. The results of the study show that the virtual experiments compare well with a physical experiment, and that such virtual experiments are a powerful tool to examine the accuracy of velocity gradient measurements. The cross-stream gradients needed to determine the vorticity components can be measured with sufficient accuracy with most of the array and sensor configurations of vorticity probes used so far. A systematic error of some of the gradient measurements can appear due to the array or sensor configurations. None of the examined probe designs can measure, with sufficient accuracy, the stream-wise velocity gradients, directly or indirectly, using the continuity equation.