# Universality in quasi-2D granular shock fronts above an intruder

## TL;DR

This study experimentally investigates quasi-2D granular flow around intruders, revealing universal shock front shapes described by an inverted catenary, and links flow non-uniformities to force interactions.

## Contribution

It demonstrates the universality of shock front shapes in granular flow and connects flow field measurements to force dynamics on intruders.

## Key findings

- Shock fronts are well described by an inverted catenary shape.
- Flow non-uniformities are linearly related: temperature, divergence, shear rate.
- Intruder shape and impact speed scale and shift the shock front without altering its form.

## Abstract

We experimentally study quasi-2d dilute granular flow around intruders whose shape, size and relative impact speed are systematically varied. Direct measurement of the flow field reveals that three in-principle independent measurements of the non-uniformity of the flow field are in fact all linearly related: 1) granular temperature, 2) flow field divergence and 3) shear-strain rate. The shock front is defined as the local maxima in each of these measurements. The shape of the shock front is well described by an inverted catenary and is driven by the formation of a dynamic arch during steady flow. We find universality in the functional form of the shock front within the range of experimental values probed. Changing the intruder size, concavity and impact speed only results in a scaling and shifting of the shock front. We independently measure the horizontal lift force on the intruder and find that it can be understood as a result of the interplay between the shock profile and the intruder shape.