## 6 Prestall Disturbances  
In the next step, different analysis methods are presented in order to quantify the development and intensity of prestall disturbances. The primary objective is to obtain a representative scalar value that gives a clear indication of the beginning of prestall disturbances as the flow coefficient is reduced. Data processing was performed at specific operating points which have been already shown in the total-to-static pressure rise characteristic. The preceding analysis of the blade passing signature by phase averaging has been carried out at a constant flow coefficient. In the following, the root mean square (RMS) calculation was applied on the difference between the average trace and the signal of the first revolution. After repeating this process for 100 revolutions, the mean of all RMS was nondimensionalised by the dynamic pressure at the rotor inlet. A detailed description of the presented analysis procedure is given by [PERSON] et al. (2011) [5]. It is now possible to determine the variance of pressure fluctuations caused by the disturbances themselves. In order to be consistent with the well established nomenclature of prestall instabilites, this special evaluation of RMS is called irregularity. The level of irregularity is computed for the same operating points as shown in Figure 3 and plotted against the flow coefficient. For both speeds, a clear ramp-up in irregularity can be observed when going from open throttle position to stall (see Figure 5a). Especially at near stall, the intensity of prestall disturbances increases disproportionately. At the last stable operating point, the amplitude of fluctuations induced by the disturbances is nearly 0.5% of the dynamic pressure at the rotor inlet. Regarding the fact that the disturbances are characterised by a sharp pressure drop, the third statistical momentum (skewness) seems to be suitable for analysing the inception process of prestall instabilities. If the majority of pressure fluctuations is higher than the average or the signal contains drops of very low pressure respectively, the skewness is negative [14]. Hence, evidence of prestall disturbances is indicated by negative values of the skewness. Equal to the RMS calculation, the skewness is applied to the difference between the average trace of the blade passing signature and every individual rotor revolution. In an operating range where the flow coefficient is higher than \(\varphi=0.49\), the skewness is almost zero--no prestall disturbances are detectable (see Figure 5b). The inception of prestall instability is indicated by a significant drop of the skewness at \(\varphi<0.49\). The flow becomes non-uniform. The intensity of negative pressure spikes increases as the compressor approaches stall. Nevertheless, it should be noted that the skewness just gives a rough idea of when the inception process of flow disturbances is initiated. Figure 4: Filtered pressure fluctuations at B18; Difference between mean blade passing (black line in Figure 3b) and the first revolution (one exemplary red line). [PERSON] pointed out that the level of irregularity depicted in Figure 5a does not allow a reliable stall warning because the ramping up trend changes due to compressor degradation over life time. As a result, it is not appropriate to define a constant alarm level in the distribution of irregularity. In contrast, a new stall indicator now represents an unmistakable parameter which can be used to announce the stalling limit even at transient operation (not shown here). For this purpose, the third statistical momentum has been applied on the unsteady pressure data without excluding the blade passing signal. The skewness is evaluated at several steady operating points between open throttle and stall. In the following text, the skewness of the unprocessed pressure signal measured 3 mm upstream of the leading edge plane is called \"stall indicator\". It is displayed in Figure 6 as an averaged value from both fast response pressure transducers and plotted against the flow coefficient (circles).